1. Field of the Invention
This invention relates to a rewritable-type information recording medium applicable to an optical memory element, an optical disk and a filing system for documents.
2. Discussion of Background
Rewritable-type information recording media enable the user to erase unnecessary information and write new information as many times as desired by applying electromagnetic waves.
Conventionally known recording media of this type are as follows: A rewritable-type information recording medium which has a recording layer made of an amorphous film of a rare earth metal and a transition metal, and utilizes the magneto-optical effect, as reported by Shin Uchiyama in "Surface Chemistry", Vol. 8, page 2 (1987), and a rewritable-type information recording medium which has a recording layer made of a thin film of a chalcogen alloy, and utilizes a change between a crystalline phase and an amorphous phase, as reported by Masatoshi Takao in "Electro-Ceramics", November, page 16 (1987).
It is however difficult to overwrite on the former recording medium by applying a single beam thereto. Moreover, it requires an extremely complicated optical system on the driving side, so that the total cost becomes high.
On the phase-change-type information recording medium, information can be overwritten by applying a single beam. Moreover, the recording medium of this type does not require a complicated optical system on the driving side. For these reasons, research and development of such a recording medium is extensively being made nowadays.
Thin films of Ge-Te, Ge-Te-Sb, Ge-Te-S, Ge-Se-S, Ge-Se-Sb, Ge-As-Se, In-Te, Se-Te and Se-As, disclosed in U.S. Pat. No. 3,530,441, are known as typical chalcogen alloy thin films usable as the recording layer of the phase-change-type information recording medium.
In addition, aiming at improving the stability and achieving high-speed crystallization, the following proposals have been made so far. Namely, a Ge-Te alloy thin film incorporated with Au, as disclosed in Japanese Laid-Open Patent Application 61-219692, a Ge-Te alloy thin film incorporated with both Sn and Au, as disclosed in Japanese Laid-Open Patent Application 61-270190, and a Ge-Te alloy thin film incorporated with Pb, as disclosed in Japanese Laid-Open Patent Application 62-19490.
In order to obtain an improved recording medium which can endure repeated information recording and erasing, a material composed of Ge, Te, Se and Sb with a specific composition ratio has also been proposed in Japanese Laid-Open Patent Application 62-73438.
None of the above recording media, however, have all the characteristics required for a phase-change-type information recording medium. The remaining problems to be solved are improvement of the recording and erasing sensitivities, prevention of lowering of the erasion ratio, which is caused due to incomplete erasion at the time of overwriting, and prolongation of the expected life span of a non-recorded area.
In order to prevent the lowering of the erasion ratio, an information recording medium has been proposed, in which a phase changeable material is dispersed in a light-transmittable matrix. Agglomeration of crystals of the phase changeable material, which causes the lowering the erasion ratio, can thus be successfully prevented.
As the light-transmittable matrix, silicon oxide, aluminum oxide, titanium oxide and magnesium oxide are proposed in Japanese Laid-Open Patent Application 57-208648. Furthermore, Japanese Laid-Open Patent Application 63-173240 discloses, as the matrix metal oxides, metal nitrides, metal sulfides and metal carbides having low thermal conductivity, such as SiO.sub.2, SiO, Si.sub.3 N.sub.4, TiO.sub.2, ZnS, ZnO, Al.sub.2 O.sub.3, AlN, MgO, GeO, SiC, ZrO and Nb.sub.2 O.sub.5.
In the case where any one of the above-mentioned inorganic material is employed as the matrix, crystal nuclei are formed and grow when information is repeatedly recorded and erased. As a result, the recorded information cannot be erased completely, and the C/N ratio is lowered due to a granular noise.
Organic materials can also be used as the matrix, as disclosed in Japanese Laid-Open Patent Applications 60-124038, 63-205832 and 63-206921. According to these patent applications, information recording media are prepared b simultaneous sputtering of Te and a heat-resistant resin. In addition, an information recording medium prepared by reactive sputtering of Te in CH.sub.4 is reported in Technical Report CPM83-58 in the Institute of Electronics and Communication Engineers.
It is, however, extremely difficult to control the particle size and the dispersing state of Te when the above methods are employed. Moreover, when the organic matrix is prepared by an electric discharge, a reaction pressure is adjusted to approximately 10.sup.-2 Torr. Under such a pressure, coagulation of the dispersed material particles of Te is inevitable. It is therefore quite difficult to obtain a recording layer made of the organic matrix in which finely-divided particles of Te are dispersed.
An attempt has also been made to employ as the matrix a film prepared by plasma polymerization. In general, however, a plasma-polymerized film tends to contain a component which vaporizes at a temperature of 300.degree. C. or less. Therefore, such a film is suitable for an overwritable-type recording medium, but unsuitable for a rewritable-type recording medium.
A recording layer of an information recording medium is readily deteriorated by moisture and oxygen contained in the air, and light. For this reason, one or both surfaces of the recording layer are generally provided with a protective layer. In addition, when information is recorded in the phase-change-type optical recording medium, the recording layer is melt due to heat applied thereto. It is therefore preferable to provide a protective layer to the recording layer.
The protective layer may be prepared by a metal oxide, a metal nitride, a metal sulfide or a metal carbide, such as SiO.sub.2, Si.sub.3 N.sub.4, ZnS, AlN, SiC or ZrO.sub.2. A mixture of ZnS and SiO.sub.x in which x is 1 to 1.8 can also be used for the protective layer as disclosed in Japanese Laid-Open Patent Application 63-276724.
However, when any one of the above metallic compounds is employed as the protective layer, crystal nuclei are formed and grow when information is recorded and erased repeatedly. As a result, the recorded information cannot be erased completely, and the C/N ratio is also lowered due to a granular noise.
Furthermore, in the case where a recording layer made of a chalcogen film containing Te is employed in combination with the protective layer made of the above metallic compound, these two layers tend to react with each other to yield a metallic chalcogen compound. For this reason, the C/N ratio is lowered and the expected life span of the recording medium is also shortened.
Recently, an information recording medium having a heat-radiating layer has been proposed. Such a recording medium can be used for high-speed information recording and erasing.
Either a thin film, as disclosed in Japanese Laid-Open Patent Application 63-244423, prepared by dispersing metal powder of Al, Au or Cu having a particle size of 100 to 200 .ANG. in a dielectric substance such as SiO.sub.2, TiO.sub.2, Si Ta.sub.2 O.sub.5 or ZnS, or a metallic thin film, as reported in "1962 Autumn Symposium--Japanese Journal of Applied Physics", 18p, 2p, 12, page 736, which can also serve as a reflective layer, is used as the heat-radiating layer.
The above-described metals and metallic compounds tend to yield crystal nuclei to grow when information recording and erasing are repeated. As a result, the recorded information cannot be erased completely, and the C/N ratio is also lowered due to a granular noise.
Moreover, the heat-radiating layer is used together with a recording layer made of a chalcogen containing Te, these two layers react with each other to yield a metallic chalcogen compound. Thus, the C/N ratio is lowered, and the the expected life span of the recording medium is shortened.
In general, when the thermal conductivity of the heat-radiating layer is too low, the quenching effect cannot be obtained. On the contrary, the thermal conductivity is too high, heat cannot be effectively utilized, resulting in lowering the recording and erasing sensitivities. It is therefore necessary to control the thermal conductivity of the heat-radiating layer adequately to the recording layer. It is however quite hard to control the thermal conductivity of the above-described materials usable for the heat-radiation layer.